CLIMATE. 



CLIMATE. 



Abw, to iucline) *eenu originally to hare been used to express tho 

 apiumit inclination of the heavens toward* the horizon ; and it was 

 afterwards used as a technical term in astronomy and geography to 

 indicate generally the dirtanoe on the celestial iphere, aa well as on the 

 terrestrial globe, from the equinoctial line towards the poles. 



The plural of this word, nfdiutra (climata), however, was used in n 

 omewhat different sense, and answered, in some degree, to our paral- 

 lels of latitude. The Greek* supposed the celeatial sphere to be 

 divided into girdle* or zones, parallel to the equator, and surrounding 

 the whole sphere. They then endeavoured to ascertain through what 

 stars the parallel circles forming the boundnry-linea ofjsuch a zone were 

 to be drawn, and what stars fell within the zone. The same division 

 they afterwards applied to the terrestrial globe, ascertaining first the 

 place* through which the boundary-lines of the zones ran, and then 

 determining at what ^i-fam~ from them the intermediate places were 

 situated. These zones were called climata, or climates, and were used 

 by them as we now use the degrees of latitude. 



These climata of the Greek geographers however did not always 

 embrace an equal number of degrees of latitude. The principle of the 

 division into zones was the length of the longest days ; and a differ- 

 ence of *!* an hour generally determined the breadth of one of these 

 climates. For instance, one of the boundary-lines of a zone would run 

 through the places in which the longest day had thirteen hours, and 

 the other through those in which it was thirteen hours and a half. 

 This w.is sufficient for their purposes in those ports of the globe where 

 there was only a small number of places the position of which had been 

 determined. But it was not sufficient for those parts where the 

 number of places determined was greater, as in Greece and the adja- 

 cent countries. Here they gave a less width to the climate, allowing 

 only a quarter of an hour for the difference between the boundary- 

 lines of a climate. For instance, the southern boundary-line of such a 

 zone would run through the places whose longest day was fourteen 

 hours, and the northern through those in which it was fourteen hours 

 and a quarter. 



The greater exactness of modern astronomical observations has 

 rendered this division of the globe useless, and we have substituted for 

 it the notation by degrees of latitude. The Greeks, of course, made 

 use of these climata as we do the degrees of latitude, to indicate in a 

 general way the comparative temperature which a country enjoys by 

 reason of its smaller or greater distance from the equator. Modern 

 nations have adopted the term climate, but with somewhat enlarged 

 signification. Climate, with us, not only implies the greater or less 

 degree of heat which a country is subject to, but also the quantity of 

 moisture in its atmosphere, and, in general, all the phenomena 

 which affect vegetation, and render a country a fit abode for men and 



Heat and moisture, properly speaking, constitute climate ; but the 

 latter is dependent on the former, while that itself varies originally 

 according to distance from the equator, and height above the level of 

 the sea, the results of both co-ordinates being themselves subject to 

 variation from other causes. The other phenomena, such as win. Is. 

 electricity, *c., affect these constituents of climate ; but they require 

 a separate consideration, as modifiers of it, according to the view which 

 we take of the subject. 



\\V may observe, that those places where a high temperature is com 

 bined with a great quantity of moisture, are the most fertile, and 

 display the most luxuriant vegetation, provided the surface is not 

 formed of naked rocks ; but even the solid rocks will in time yield to 

 the combined influence of beat and moisture, and be clothed with a 

 rich vegetation ; though, on the other hand, in temperate climates at 

 least, certain igneous rocks, of which those of the extinct volcanic 

 region of Central France are examples, remain unducom posed and 

 almost devoid of vegetable covering for an indefinite period. And on 

 the Peak of Teneriffe, the lavas resulting from countless ages of volcanic 

 action, have not sensibly disintegrated ; here, however, the atmosphere 

 is arid, and the effective clouds are far below. 



The latitude of the place is generally considered as the principal 

 circumstance in determining its temperature. It is laid down as a fact, 

 that countries lying under the equinoctial line or close to it are subject 

 to the greatest comparative heat, which constantly decreases with the 

 increase of distance from that line. This general rule must be admitted 

 to be true for all the countries which lie between the tropics and tin 

 pole ; but it may be questioned how far it is true of the countries 

 within the tropics. A glance at a globe or sphere shows that the sun 

 at the end of the first month after the equinox has already advanced 

 12* of latitude towards the tropic ; but in the second month it traverses 

 only 8*. At the end of the second month it is consequently 20 from 

 the equator. There remain therefore only 3} to be traversed in the 

 third month. The sun recedes from the tropic in the same way. It 

 pass PS the first month through 84* ; the second through 8 ; and the third 

 through 12* of latitude. Hence it is evident, that at all places between 

 20* and 234* of latitude, the solar rays during two months fall at 

 noon either perpendicularly, or at an angle which deviates from a right 

 angle only by 34* at most If we take a place intermediate between 

 2O- and 2:14 ' of latitude, the solar rays must fall on it (luring tw< 

 whole months, either p*fp*adt8allrtjr, or in a direction still less re- 

 ! fp.ni the ]K-rpendicular than in the former case. On the other 

 hand, when the sun puw* the equator, two places on which the 



vertical rays of the sun fall on two consecutive days are nearly 4 of 

 atitudo dint-nit from one another ; .m.i a place situated exactly nn.l.-r 

 the equator has only during six days the sun as near its zenith as the 

 above-mentioned places near the tropics have it during two whole, 

 months. We might therefore presume, that the summer heat of the 

 alter piwitiou must be much greater than that of places near the 

 equator. This degree of temperature must be increased by the greater 

 length of the longest days, which near the tropic are 134 hours ; but at 

 the equator are always of the length of 12 hours. 



This reasoning is not contradicted by experience. The countries in 

 which the greatest degree of heat is experienced in the northern hemi- 

 sphere, lie near the tropic of Cancer. They are the countries on the 

 tank/; of the Senegal, the Tehama of Arabia, and Mekron in 1 ;.]<>- 

 chistan ; tropical Africa, however. Ueing the hottest region of all. The 

 ancients were not unacquainted with these facts ; and one of their most 

 ingenious inquirers, Posidonius, was so struck by the peculiarities of 

 the countries near the tropic, that, for the purposes of j.: 

 geography, he wished to consider them as forming a particular 

 different both from the equatorial zone and the temperate zone, and 

 separating these two in the form of a narrow belt. He observes that 

 these countries are characterised by the aridity and sterility uf their 

 soil, and that no rain falls there, while the regions nearer the equator, 

 having abundance of rain and moisture, are exceedingly fertile. 

 (Strabo, ii.) 



Though the mean temperature differs greatly in countries lying in 

 different degrees of latitude, the most intense degree of heat which is 

 experienced in all countries between the equator and 60 of latitude is 

 nearly equal. The thermometer rises almost every year at St. I 

 burg to above 90 ; and it is observed that even on the coast of Guinea, 

 and on the banks of the Senegal, it rarely exceeds 95. There are 

 certain instances in which it has attained a much higher degree. Dr. 

 Coulter observed it at 140 on the banks of the Rio Colorado (32 3(C 

 N. hit.) ; but such exceptions must be ascribed to local circum.-- 

 especially to the reflection of the solar rays from on arid and sandy 

 surface. 



The general rule, derived from geographical position, as to the distri- 

 bution of heat over the surface of the earth, is subject t many 

 exceptions, arising from local circumstances. But none of these local 

 circumstances probably affect it so much as the elevation of the surface. 

 It is a well-known fact, that near the tropics and the equator there are 

 mountains which, owing to their great elevation, are covered with 

 snow all the year round. The heat experienced in a given place not 

 only depends on the greater or less obliquity of the rays of the MIII, 

 but also on the greater or less column of the atmosphere. The column 

 of air is greatest on the surface of the sea and in such countries as are 

 nearly on a level with it. The higher we rise above this level the. 

 more the air is rarefied, and the degree of heat due to the solar rays 

 decreases. Thus we at last arrive at an elevation where the heat, even 

 under the equator, is insufficient to melt the snow. As the density of 

 the air and the intensity of the heat continually decrease aa we riw 

 higher, some attempts have been made to determine the law of the 

 decreasing temperature. Alexander von Humboldt, who made a great 

 number of observations on the steep declivities of the Andes near the 

 equator, came to the conclusion that thethermometerofFalinnli.il 

 descends one degree when we rise 343 feet above the level of the sea, 

 and one degree for every 343 feet more. Thus the thermometer may 

 be used to determine in a rough way the heights of mountains or of 

 elevated plains. The calculations of Humboldt, however, are only 

 founded on observations made in the intertropic.il countries, anil it i.s 

 supposed that the same law will not be applicable in all its e.\t 

 places situated without the tropics. Dr. Joseph Daltou Hooker, K. I!.S.. 

 from contemporaneous observations made in East K 

 28, altitude 10,385 feet, and at Calcutta, found 1 Fahr. for every 

 309 feet of elevation. 



As already observed, at a certain height above the surface of the sea 

 the heat caused by the solar rays is too feeble to melt the snow ami 

 ice. This limit has been called the snow-line, or line of pt-i ; 

 snow, or line of perpetual congelation. This snow-line does not occur 

 in all places at the some elevation, but is dc]xmdeut on the mean tem- 

 perature in summer. In warm countries it is consequently found at a 

 much higher elevation than in cold countries. It has been ascertained 

 by numerous observations, that in the Andes of South America, near 

 the equator, the summit of a mountain rising to less than 16,000 feet 

 does not attain the snow-line ; but in Norway, in 76 of lat., mountains 

 with an elevation of 2900 to 8350 feet are always covered with .-now, 

 as Professor James Forbes has shown ; an.l in tin- southern h. mi-pin ie 

 the line of periwtual snow in the Strait* of Magalhaens (5i! S 

 about 8390 feet above the sea. Nowhere in the Northern Hemisphere 

 does the snow-line descend to the level of the sea, the intensity of 

 the summer heat during the period of perpetual day effectually thaw- 

 ing the soil, though only to a trifling depth, and mixing upon its .- 

 a certain amount of brief vegetation suitable for the support of arctic 

 animals. In the Southern Hemisphere, however, the tempera' 

 which, as a whole, is greatly inferior to that of the Northern, the MIOU- 

 line attains the level of the sea itself (according to Sir James C. Ross, 

 as cited '> Professor Forbes) in the Antarctic regions, at a latitude 

 between 67 and 71, under which, in Norway, forests still grow, and 

 even corn in some sheltered places. Numerous observations uiiidc on 



